The phase-locking mid-pulse detector may be used in a receiver to detect the exact time of arrival of a pulse.
The phase-locking mid-pulse detector operation is based upon the principle that the instantaneous phase reversal of a carrier frequency can be readily detected as a zero-crossing of the amplitude modulation function, or waveform. In more detail, let a(t) represent the desired modulation function and let cos (2.pi.f.sub.c t+.theta.) represent the carrier, where .theta. is merely an arbitrary phase factor. The modulated carrier, a(t) cos (2.pi.f.sub.c t+.theta.), is the product of the two. The finite bandwidth of the signal generator, transmitting system, and the transmission path, introduces finite rise and fall time effects.
In the prior art, leading edge detection has been used to determine pulse arrival time. This method is subject to biased errors which are a function of signal level relative to detection threshold. The mid-pulse detector of this invention detects a zero-crossing of the modulation function and eliminates this sensitivity to signal amplitude.
Another prior art method of mid-pulse detection is the pulse-differentiation technique. The pulse is first heavily filtered so as to produce a rounded pulse envelope. The differentiation process then produces a zero crossing at the peak of the pulse. This method is sensitive to pulse envelope distortions, which cause the pulse peak to shift relative to the true mid-pulse. Here again, the phase-locking mid-pulse detector has the advantages of being insensitive to pulse-envelope modulation.